The present invention relates to a ceramic diaphragm structure and a method for manufacturing the ceramic diaphragm structure. Particularly, the present invention relates to a ceramic diaphragm structure having a specific configuration of a thin diaphragm portion and a method for manufacturing such a ceramic diaphragm advantageously.
There has conventionally been used a diaphragm structure as a member or the like constituting various kinds of sensors. Attention has recently been paid to the diaphragm structure as a member constituting a piezoelectric/electrostrictive actuator. The diaphragm structure includes a substrate having at least one window portion and a thin diaphragm plate formed of flexible, filmy material covering the window portion. When the diaphragm structure is used as a member constituting a sensor, the diaphragm structure is constituted so that a bending displacement caused by an object to be measured by a diaphragm portion is detected by an appropriate means. When the diaphragm structure is used as a member constituting a piezoelectric/electrostrictive actuator, the diaphragm structure is used under the condition that the diaphragm portion is transformed by a piezoelectric/electrostrictive element, causing a pressure in a hollow portion formed inside the diaphragm structure.
Such a diaphragm structure is produced by unitarily combining a substraite with a filmy member which gives a diaphragm. In view of a reliability, thermal resistance, anti-corrosive ability and the like, it was considered to constitute such a diaphragm structure by unitarily firing a ceramic. The present inventors have disclosed in Japanese Patent Laid-Open 63-292032 and Japanese Patent Laid-Open 5-49270 a pressure detecting apparatus or a piczoelectric/electrostrictive actuator, in which a ceramic diaphragm structure obtained by unitarily firing was used.
Such a ceramic diaphragm structure is generally obtained by firing a unitary laminate composed of (1) a ceramic green substrate having a predetermined configuration and a window portion and (2) a thin ceramic green sheet covering the window portion. However, the present inventors found that there are some problems during a unitary firing operation that a diaphragm portion, which is formed of a ceramic green sheet and positioned on the window portion of the ceramic green substrate, deforms into a depressed shape (The shape that a diaphragm portion protrudes in the opposite direction of the window portion is a protruded shape.) or has cracks. Such a depression or a crack in the diaphragm portion hinders a function or an operation of the diaphragm and makes the reliability of the diaphragm deteriorate.
Additionally, in the case of such a ceramic diaphragm structure, it is usually tried to make a diaphragm portion plane. However, in such a plane diaphragm portion, there are some problems. It is difficult to increase the resonance frequency and to thin the portion because the strength is not satisfactory. Further, an electrode film or a piezoelectric/electrostrictive film formed on the surface cannot be satisfactorily sintered.
Therefore, the present inventors have previously disclosed a thin ceramic diaphragm structure having a high reliability in Japanese Patent Application 6-122733. The diaphragm structure is free from a depression, a crack, or the like in a diaphragm portion, and a resonance frequency of the diaphragm can be increased. Further, the diaphragm structure is excellent in strength, and various films formed on the surface of the diaphragm portion can be satisfactorily sintered without being hindered.
The ceramic diaphragm structure was produced by controlling a difference of sintering speed and a firing shrinkage rate between a ceramic green substrate and a thin ceramic green sheet covering a window portion. However, when a substrate is composed of many structures each having a plurality of window portions, a diaphragm portion sometimes has a depression or has a variance of the height of protrusion.